Quenching of positronium in nitrogen dioxide

The quenching mechanism of positronium in N${\mathrm{O}}_2$ was investigated by measuring both positron lifetimes and two-photon angular-correlation curves of annihilating positron-electron pairs in N${\mathrm{O}}_2$ adsorbed on silica gel. It is found that instead of a conversion reaction by N${\mathrm{O}}_2$, chemical quenching is responsible for the quenching of positronium. The quenching cross section for N${\mathrm{O}}_2$ is compared with the conversion cross sections of ${\mathrm{O}}_2$ and NO gases and also with the chemical quenching cross sections of ${\mathrm{Br}}_2$ and ${\mathrm{I}}_2$ molecules. The cross sections for chemical quenching are at least two to three orders of magnitude larger than the cross sections for conversion quenching of Ps. The bond strengths of these molecules are found to play an important role in Ps quenching process.